The present invention relates to a method for electronically determining process variables in a compounding process and more particularly the present invention relates to a method of determining the concentration of ingredients in a compounding process by measuring the capacitance and the dissipation factor of the compounded mixture.
As noted, in the publication of Hedwig, entitled "Dielectric Spectroscopy of Polymers," published by John Wiley & Sons, 1977, and especially Chapter 6 and 7 of that book, there is disclosed that various types of organic polymers have determinable measurable dielectric constants and dissipation factors which dielectric constants and dissipation factors vary with the degree of polymerization of the polymer. Also, there is the publication of Sanjana, entitled, "The Use of Dielectric Analysis in Characterizing the Degree of B-Staging and Cure of Composites,"--International Conference on Polymer Processing, August 1977, MIT Press (1978), which is concerned with the determination of the B stage of a thermosetting polymer by measuring the dielectric dissipation factor of the mixture. However, nothing was mentioned in these publications about determining the processing variables of thermoplastic polymers or the ingredients input of thermoplastic or thermosetting polymers and determinations of the degree of polymerization of the final desired polymer. In the copending case, Golba et al., Ser. No. 374,585, which was filed on the same date as the present case and entitled "Method for Measuring Polymerization Process Variables," there is disclosed a method for determining the concentration of ingredients in a continuous polymerization process or in a batch polymerization process by determining the dielectric constant and dissipation factor of the monomer mixture with various additives in it, as it passes from one processing step to another. Thus, first the capacitance of the mixture is measured in e.g. a capacitance cell then translated to a dielectric constant which by computer means or otherwise can be utilized to determine the concentration of ingredients from a calibration curve. In an analogous way, the catalyst and a stabilizer additive can be determined by measuring dissipation factor and capacitance and utilized to control the process. Further, the capacitance of the mixture can be measured in various stages of polymerization and translated to a dielectric constant and the dissipation factor can be measured which, by use of computer means or otherwise, can be utilized to determine the degree of polymerization of the mixture and used to control the process variables. Further, as disclosed in that application, the capacitance and the tan .delta. or dissipation factor of the monomer mixture can also be measured to determine the ionic impurities in the monomers.
Accordingly, this method can be utilized to measure and control the variables such as ingredients and polymerization conditions in a batch or continuous polymerization process. This method is much preferred for continuous polymerization process since the method is electronic and can determine concentrations by use of an automatic capacitance bridge and a computer in a matter of fractions of a second. This information can then be utilized accordingly for control of the process variables. Although the method of Ser. No. 374,585 could be utilized to control the process variables for any batch or continuous polymerization process, in the preferred embodiment, it is utilized in the polymerization of polycarbonates and polyetherimides. There are other processes where it is desirable to measure and control the processing variables other than a polymerization process.
Accordingly, in the formation of thermoplastics, one important area is a compounding process. That is, for various reasons, two or more polymers are compounded into an intimate thermoplastic mixture into which various types of additives may be added. An intimate uniform mixture is desired in that it has properties which either one or both or more of the ingredients do not individually have. Accordingly, it would be highly desirable to be able to measure and continuously control the process variables in such compounding process. Once such compounding process is in the fabrication and preparation of rubber-modified styrene resin-polyphenylene ether resin compositions wherein the rubber component is of the unsaturated type such as polymers and copolymers of butadiene hereinafter also referred to as "PPO-polystyrene blend". In the past, these resins were prepared by mixing polystyrene resin with polyphenylene oxide resin and various additives, including a flame retardant such as a liquid triarylphosphate ester compound an example of which is produced by the FMC Corporation under the trade name Kronitex-50.RTM.. The mixture was mixed in a screw extruder and a sample of the uniform mixture was taken out of the extruder and analyzed for the various ingredients. If they were in proper proportions as required to generate specified property profiles, then the process would be continued to produce the desired product. It should be noted that this type of compounding process was utilized since the resin blend was a custom blend for a particular customer and various customers required different types of custom blends. In addition, in a custom blend, it was necessary to determine the ingredients coming out of the extruder to determine that indeed, the desired end composition of compounded product was obtained from the extruder before the material could be further produced to meet a customer's needs.
Up to this time, the standard procedure for the analysis of such blends was to take a sample of a compounded blend and submit to x-ray fluorescent spectroscopy or other analytical means, determine the concentration of ingredients in the laboratory and then utilize the results to monitor the inputs of ingredients into the compounding process. Such analyses are time consuming and resulted in undesirable downtime in the compounding process. Accordingly, it was highly desirable to have an on-line/real time means for determining the ingredients in a compounding process such that the process could be operated in a batch or in a substantially continuous manner.